In semiconductor fabrication it is desirable to employ automated handling on the semiconductor wafers for efficient processing. Said wafers are typically retrieved from and loaded back into a carrier holding a number of wafers. The space available between adjacent wafers in the carrier is very small.
Insofar any handling apparatus must be constructed with a flat and elongated equipment to be fed in or retracted from said carrier.
At the same time, while the wafers are being moved, the wafers must be held securely, but gently and are not allowed to interfere with adjacent wafers in the cassette (carrier). Any contact with the surfaces of the wafers should be avoided.
U.S. Pat. No. 5,711,646 discloses a substrate transfer apparatus for receiving and delivering a substrate among a plurality of process locations and transferring the substrate among the process locations. Said apparatus comprises one transfer member provided with two support portions, one for supporting the wafer in a first position and second support portions for supporting the wafer in a second position. With this apparatus no exact placement of the wafer within the transfer member may be achieved and a contact between the wafer surface and the transfer member may not be avoided with certainty.
U.S. Pat. No. 4,900,214 provides an apparatus of the kind mentioned above with an elongated arm having a first jaw and a second jaw for corresponding engagement of the edge of a wafer. Insofar the same disadvantages appear as with respect to the apparatus known from U.S. Pat. No. 5,711,646.
An apparatus with piezoelectric grippers is known from EP 0 810 636 A2.
EP 0 778 611 A2 discloses an end effector for a wafer transfer device, comprising a pocket allowing the end effector to contact only the edges of the wafer.
A similar device is described in EP 0 555 890 A2.
Again a wafer pick up device with just one carrier arm is disclosed in U.S. Pat. No. 4,639,028.
The semiconductor slice holder according to U.S. Pat. No. 5,022,695 has three tines, a center tine and two side tines. The side tines are fixed to position, while the center tine is moved in and out using a control field actuator. A Hall effect sensor on the controlled field actuator provides an electrical feedback to provide a firm gripping force with reduce damage to slice edges by sensitive control of the gripping force.
Another apparatus of the kind mentioned above is presented by U.S. Pat. No. 4,813,732, comprising three rod-like projections (called fingers), one finger of which is extendable, while the others are fixed. Button shaped tips located on the ends of each of the fingers serve to engage the wafers edge.
Most of said devices are based on the so called ECO principle (ECO=Edge Contact Only). Said ECO does not entirely prevent contamination issues. These edge contact points also become susceptible to contamination if using the same contact points for handling and transport before and after wafer process steps. Wafers using the same contact points after processing can become re-contaminated thus causing degradation of the semiconductor devices.